1. Field of the Invention
The present invention relates to a thermal infrared solid-state imaging device for detecting a temperature change generated by an incident infrared ray with two-dimensionally arrayed semiconductor sensors, and more particularly to a thermal infrared solid-state imaging device that integrates electric signals from the semiconductor sensors in a signal processing circuit to outputs a detection signal.
2. Related Art
A general thermal infrared solid-state imaging device includes pixels, each having a thermally insulating structure, which are arrayed two-dimensionally, and captures an infrared image by using a change in temperature of pixels caused by an incident infrared ray. Regarding a non-cooling type thermal infrared solid-state imaging device, as a temperature sensor serving as a pixel, several sensors are known, such as a bolometer which is made of polysilicon, amorphous silicon, silicon carbide, vanadium oxide, or the like, or a semiconductor device such as diode or transistor, and so on. In particular, the semiconductor device such as diode has small fluctuation in electric characteristic and temperature dependency among devices, and is very advantageous for improving the uniformity of characteristics of individual pixels.
In the thermal infrared solid-state imaging device, pixels are arrayed two-dimensionally, and each of rows is connected via a drive line, and each of columns is connected via a signal line. Each drive line is sequentially selected by a vertical scanning circuit and a switch, and power is supplied to the pixel from a power source by way of the selected drive line. The output of the pixel is transmitted to an integrating circuit by way of a signal line, and is integrated and amplified in the integrating circuit, and is sequentially output to an output terminal by means of a horizontal scanning circuit and a switch (see, for example, “Low-cost 320×240 non-cooling IRFPA using conventional silicon IC process”, Ishikawa et al., Part of the SPIE Conference on Infrared Technology and Applications XXV, published April 1994, Vol. 3698, pages 556-564).
In such a thermal infrared solid-state imaging device, voltage drop in the drive line affects the voltage supplied into the integrating circuit, in addition to the voltage across the pixel, so that amount of the voltage drop in the drive line differs in each pixel column. As a result, the output of the integrating circuit is different in each pixel column, so that an offset distribution due to resistance of a drive line occurs in a captured image. Besides, the response to infrared light of the thermal infrared solid-state imaging device, that is, the change in the voltage across the pixel is far smaller than the voltage drop component in the drive line. Accordingly, the amplifier may be saturated by the voltage drop distribution by the drive line, and a necessary degree of amplification may not be achieved.
Moreover, the response of the pixel includes a response due to a device temperature change aside from the response of infrared light, and thus the device output may drift along with the device temperature change. In other words, it is ideal that the pixel is completely insulated from heat so that only the temperature change due to infrared absorption can be detected, however the heat insulation structure of the pixel has a finite heat resistance, and thus the output changes if the ambient temperature varies during detection operation. Since the output variation due to this ambient temperature change cannot be distinguished from the change in the incident infrared ray, the measurement precision of infrared ray is lowered, and a stable image cannot be obtained.
To solve the problems, the thermal infrared solid-state imaging device disclosed in JP2005-214639A has the following structure. Specifically, as shown in FIG. 13, the thermal infrared solid-state imaging device includes pixel arrays 1 disposed two-dimensionally, reference dummy pixel columns 12 composed by excluding a thermally insulating structure and/or an infrared ray absorbing structure, signal lines 23 connected to first constant current means 2 at a terminal end, bias lines 19 for connecting in parallel second constant current means 20 provided in each column of a pixel area for causing a voltage drop nearly same as drive lines 3, and differential integrating circuits 7 for integrating the difference of the voltages at both ends of the first constant current means 2 and the second constant current means 20 for a constant period of time to output the integrated signal. A sample hold circuit 13 samples and holds a reference dummy pixel output signal from the differential integrating circuits 7, compares the reference dummy pixel output signal with a reference voltage, generates a bias voltage depending on the difference, and supplies the bias voltage to the bias lines 19. By such use of the differential integrating circuits in reading of outputs and the feedback mechanism of outputs of the reference dummy pixel columns to the differential integrating circuits, it is possible to solve the conventional problems, that is, the offset distribution due to voltage drop in the drive line and the temperature drift due to device temperature fluctuations.
In the two-dimensional pixel array of the above thermal infrared solid-state imaging device, if the number of pixels is increased to large number (for example, the number of pixels over 640×480 pixels for the two-dimensional pixel array in the conventional mainstream of the thermal infrared solid-state imaging device), the following problems occur.
To realize large number of pixels while maintaining the frame rate fr, it is necessary to set the operation speed of the horizontal scanning circuit at least higher than (1/fr)/(m*n), where, m is the number of horizontal pixels and n is the number of vertical pixels of the two-dimensional pixel array which has large number of pixels. The frame rate fr is a speed of sweeping out the outputs of all pixels by the thermal infrared solid-state imaging device (that is, the performance index showing how many two-dimensional images can be displayed per second) (for example, 30 fps).
If the operation speed of the horizontal scanning circuit being set higher than (1/fr)/(m*n) exceeds the operation speed of the MOS transistors on the semiconductors composing the thermal infrared solid-state imaging device, it is necessary to reduce the operation speed of the horizontal scanning circuit.
In the conventional thermal infrared solid-state imaging device, moreover, it may be considered to reduce the offset distribution by the use of differential integrating circuit in reading of outputs, and the feedback mechanism of outputs of the reference dummy pixel columns to the differential integrating circuit. However, even though these techniques are used, the increase of the number of pixels expands the voltage drop itself in the drive line, so that it is difficult to assure the input voltage range of the differential integrating circuit more than the voltage of the increased voltage drop in the drive line.